From my experience you cannot depend on an active A at all so when my RAH was actively drinking I told my kids not to get in car with him, even if they weren't sure and felt uncomfortable. Keep in mind kids need direction so I also had to confide in some friends and ask them if they could be called if I was not available and my kids found themselves in a situation where they couldn't get in car with Dad.
You are doing what is best for your children. Unfortunately, the disease makes no accommodations for A's to do what is best for their kids until they actively manage their disease and stop drinking.
And, yes my son who was 11 at the time had an uncomfortable feeling and did not get in car with his dad and sure enough, dad drove away and ended up with a DUI.
(((HUGS))) to you. I am sorry for your pain.
As for the letter, it is great if it makes you feel better and yes I understand all that you have written but as TG said the addiction is most powerful and speaks louder.
What I found out is you end up bargaining for everything with an A. First it's don't drive with the kids in the car, drinking still continues, then it's don't talk to me when you are drinking, drinking still continues, then it's go to meetings, get some help and yes, unless the A really, really wants to stop drinking will continue even when going to meetings (although there is more of a chance that something at a meeting will reach them and lead them to stop).